Fallgren, Mikael

Abstract [en]

In this thesis we formulate joint cell, channel and power allocation problems within wireless communication networks. The objectives are to maximize the user with mini- mum data throughput (Shannon capacity) or to maximize the total system throughput, referred to as the max-min and max-sum problem respectively. The complexity is stud- ied together with proposed optimization- and heuristic-based approaches.

In the first paper an overall joint cell, channel and power allocation max-min prob- lem is formulated. We show that the decision problem is NP-hard and that the op- timization problem is not approximable unless P is equal to NP, for instances with a sufficiently large number of channels. Further, it follows that for a feasible binary cell and channel allocation, the remaining continuous power allocation optimization problem is still not approximable unless P is equal to NP. In addition, it is shown that first-order optimality conditions give global optimum of the single channel power al- location optimization problem, although the problem is in general not convex.

In the following two papers heuristics for solving the overall problem are proposed. In the second paper we consider the single channel problem with convex combinations of the max-min and the max-sum objective functions. This variable utility provides the ability of tuning the amount of fairness and total throughput. The third paper investi- gates the multiple channel setting. On a system with three cells, eight mobile users and three channels, we perform an exhaustive search over feasible cell and channel alloca- tions. The exhaustive search is then compared to the less computationally expensive heuristic approaches, presenting potential earnings to strive for. A conclusion is that several of the proposed heuristics perform very well.

The final paper incorporates fixed relay stations into the overall joint cell, channel and power allocation max-min problem. The complexity is inherited from the formula- tion without relay stations. Further, we propose a heuristic channel allocation approach that shows good performance, compared to an optimization based approach, in numer- ical simulations on the relay setting.

Fallgren, Mikael

Abstract [en]

We consider wireless telecommunications systems with orthogonal frequency bands, where each band is referred to as a channel, e.g., Orthogonal Frequency-Division Multiple Access (OFDMA). For a given snap-shot in time, the joint cell, channel and power allocation optimization problem is presented, both in downlink and in uplink. The objective is to maximize the minimum total Shannon capacity of any mobile user in the system, subject to system constraints. The corresponding decision problems are proved to be NP-hard. We also show that for any constant ρ > 0, a sufficiently large number of channels ensure that the optimization problems are not ρ-approximable, unless P is equal to NP. Furthermore, we show that the inapproximability property remains when solely considering the power allocation problem, i.e., given a feasible cell and channel allocation. This power allocation optimization problem is not convex in general, but in the simplified setting where each transmitter is allowed to use only one single channel, there exists known approaches to attain the global optimum. In this setting, we prove that any solution that fulfills the KKT conditions is a global optimum.

National Category

Computational Mathematics

Identifiers

urn:nbn:se:kth:diva-40400 (URN)

Note

Condensed version published in
IEEE International Workshop on Quality of Service
QC 20110914Available from: 2011-09-14 Created: 2011-09-14 Last updated: 2011-09-15Bibliographically approved

Abstract [en]

In this paper, we consider joint resource allocation of a multicell OFDMA-based networks, with fixed two-hop decode-and-forward relay stations. A joint cell, channel and power allocation problem is formulated as an overall optimization problem, where the objective is to maximize the minimum user throughput. Based on previous complexity results for the setting without relays, the overall optimization problem is shown not to be approximable, unless P is equal to NP. We propose a method for solving this challenging problem. First, a feasible cell allocation is obtained, either via greedy allocation or an exhaustive search. Thereafter, the channel and power allocations are alternately updated, either using a heuristic or optimization-based approach while holding the other two allocations fixed. These alternating channel and power allocations are repeated until no further improvement is obtained. The impact of relay stations is investigated by considering a model with relays as well as one without relays. The simulations show that substantial performance improvement can be made by introducing relays. They also indicate, somewhat surprisingly, that the heuristic channel allocation leads to better overall solutions than the corresponding optimization approach in the relay setting. The optimization-based power allocation significantly outperforms the heuristic power approach with as well as without relay stations.

Fodor, Gábor

Forsgren, Anders

Abstract [en]

In multicell wireless networks the resource allocation task includes the selection of the serving cell and the allocation of channels and transmission powers. While all of these tasks have been studied in the past, all three jointly are seldom addressed. In this paper we formulate the joint cell, channel and power allocation problem as an optimization task, whose purpose is to maximize either the minimum user throughput or the multicell total throughput. The max-min problem and a simplified max-sum problem are both known to be NP-hard, and we therefore propose heuristic solution approaches. In particular, we decompose the joint problem to the separate subproblems of cell selection, channel assignment and power allocation. We propose heuristic and optimization based algorithms to solve each of these tasks. An iterative channel and power update approach then address both the max-min and max-sum problems. Further, we also perform an exhaustive search over feasible cell and channel allocations. We present numerical results that give new and valuable insights into the trade off between fair and sum throughput optimal joint resource allocation strategies. Our proposed heuristics perform surprisingly close to the optimization based approaches,while the run time performance is significantly improved.

Oddsdóttir, Æsa Hildur

Fodor, Gábor

(English)Manuscript (preprint) (Other academic)

Abstract [en]

Since the seminal paper by Knopp and Humblet that showed that the system throughput of a singlecell system is maximized if only one terminal transmits at a time, there has been a large interest in opportunistic communications and its relation to various fairness measures. On the other hand, in multicell systems there is a need to allocate transmission power such that some overall utility function is maximized typically under fairness constraints. Furthermore, in multicell systems the degree of resource allocation freedom includes the serving cell selection that allows for load balancing and thereby the efficient use of radio resources. In this paper we formulate the joint serving cell selection (link selection) and power allocation problem as an optimization task whose purpose is to maximize either the minimum user throughput or the multicellsum throughput. The max-min problem and a simplified max throughput problem are both NP-hard and we therefore propose heuristic solution approaches. We present numerical results that give new and valuable insights into the trade off between fair and sum throughput optimal joint resource allocation strategies.

National Category

Computational Mathematics

Identifiers

urn:nbn:se:kth:diva-40401 (URN)

Note

Condensed version published in
IEEE International Conference on Communications
QC 20110914Available from: 2011-09-14 Created: 2011-09-14 Last updated: 2011-09-15Bibliographically approved